Field of the Invention
The present invention relates to a voice decoding apparatus, and more particularly, to a voice decoding apparatus for decoding a voice signal encoded by a Multi-Band Excitation (MBE) type voice encoding system.
Description of the Background Art
In Japan, the Radio Law has been revised because of concern as to an increase in demand of data transmission and tightness of frequencies. From the revision, a telecommunications system of a so-called convenience radio device has been determined so as to be completely shifted from a conventional analog type to a digital type. In response to such a trend, a standard relating to the telecommunications system of the digital type convenience radio device, i.e. a digital radio device, is determined by Association of Radio Industries and Businesses (ARIB). With regard to a 4-level FSK (Frequency Shift Keying) modulation system often applied to a specified low power radio device, a 4-FSK communication radio system for broadcasting business, e.g. ARIB STD-B54, is determined in a broadcasting field and a narrow band digital communication system, e.g. SCPC (Single Channel Per Carrier)/a 4-level FSK type, i.e. ARIB STD-T102, is determined in a telecommunications field. As to voice encoding system, the standards in both the fields describes that “AMBE+2 (Advanced Multi-Band Excitation plus two) Enhanced Half-Rate in Digital Voice System, Inc. is recommended”. The trade mark AMBE+2 (occasionally written as AMBE++) is held by Digital Voice System, Inc.
AMBE+2 has two advantages as compared with other voice encoding systems in that a decoded voice hardly sounds unnaturally in noisy environment and that a stable quality is provided in low bit rate. However, the Researches and Investigations Society Information, “A Research and Examination Report Relating to Common Use between a Frequency for an Analog Convenience Radio Station Using 150 MHz Band and a Frequency for Digital Type”, Hokuriku Bureau of Telecommunications, Ministry of Internal Affairs and Communications, 2011, reported that “a voice sounds like clogging a nose”. Thus, AMBE+2 has a disadvantage of degrading the quality of sound.
AMBE+2 is an advanced system based on MBE (Multi-Band Excitation) as the voice encoding system, and AMBE is aberration on Advanced MBE. In addition to AMBE, there is another voice encoding system called as IMBE (Improved MBE). AMBE or AMBE+2, and IMBE are based on MBE. In this specification, MBE, AMBE and IMBE may be called as a “MBE-type voice encoding system”. The term “MBE voice encoding system” herein indicates that MBE is used as the voice encoding system.
However, as reported by the above-mentioned Research and Examination Report, the MBE-type voice encoding system has a problem that the decoded voice becomes like clogging a nose. Hereinafter, such sound quality will be called as a “nose clogging feeling”.